1. Field of the Invention
The present invention relates to manufacturing of a semiconductor device, and more particularly to controlling the thickness of a thin film layer in the device.
2. Description of the Related Art
Manufacturing of semiconductor devices typically includes forming N or P-type regions in a semiconductor substrate by doping various impurities into the substrate, and forming various thin film layers on the substrate. Several different chemical vapor deposition (CVD) methods can form thin films. Low pressure chemical vapor deposition (LPCVD) is one of the CVD methods.
Typically, LPCVD does not require high temperature and a high pressure in forming a thin film layer. For instance, the pressure range of LPCVD processes is generally between 0.1 torr and 100 torr. LPCVD can form most thin film layers of a semiconductor device, for example, an oxide film, a polysilicon film, a tungsten film, a silicide film, a nitride film, and an epitaxial film. Further, LPCVD can form a thin film layer that has small number of particles and a good step coverage.
A typical apparatus for performing the LPCVD includes a bell-shaped outer tube, an inner tube in the interior of the outer tube, a gas supply system connected to the inner tube, a gas exhaust system connected to the outer tube, a wafer boat elevator that moves a wafer boat to and from the inner tube, and an electric furnace that is around the outer tube to maintain the process temperature in the apparatus.
In forming a thin film layer of a semiconductor device using the above-described apparatus, process conditions, such as deposition time, process temperature, and density of the gas, determine the thickness of the thin film layer. To obtain a thin film layer with a desired thickness, a monitoring wafer is loaded in the inner tube with other wafers, and the thickness of a thin film layer on the monitoring wafer is measured. Based on the measured thickness, the deposition time in a subsequent process can be adjusted to produce a thin film layer with the desired thickness.
However, the monitoring wafer measurements only control the deposition time in achieving a thin film layer with a desired thickness. However, process temperature is also a critical factor that determines the thickness of the thin film layer, and a temperature gradient in the processing apparatus can cause film thicknesses to vary from wafer to wafer. Thus, an effective monitoring of both the process temperature and the deposition time can promote the formation of a thin film layer with a uniform thickness.